Living conditions and environmental factors
The oral cavity represents a favorable ecological niche for bacteria. This is facilitated by constant moisture, a stable optimal temperature, the presence of nutrients, and a slightly alkaline pH level.
However, microorganisms have to adapt to changing conditions. They are affected by fluctuations in temperature, acidity levels, osmotic pressure, nutrient concentrations, and oxygen partial pressure.
To successfully anchor in the mouth, bacteria must possess the ability to adhere (attach) to tissues. At the same time, the body constantly eliminates some of the microbes through mechanical actions (chewing, movements of the tongue and lips) and the flushing flow of saliva.
Dynamics of formation (ontogenesis)
The process of oral cavity colonization changes throughout a person's life:
- Neonatal period. The first bacteria enter from the mother's mouth, along with air and food. Streptococci, lactobacilli, and actinomycetes become the "pioneers" of colonization.
- Tooth eruption. New niches appear — gingival sulci. The proportion of primary colonizers decreases, giving way to gram-negative anaerobes.
- Puberty. Due to hormonal changes, the number of gram-negative anaerobes and spirochetes increases.
- Adulthood and old age. The composition depends on immunity, stress, diet, and the rate of salivation. In elderly people, reduced saliva production often leads to an overgrowth of Candida albicans fungi, which increases the risk of candidiasis.
Composition and localization of the microbiota
Microorganisms are unevenly distributed throughout the oral cavity. They prefer areas that are poorly washed by saliva: dental plaque, the dorsum of the tongue, interdental spaces, dental necks, and carious cavities. It is there that dental plaque forms — a complex, dynamic microecosystem (biofilm).
The dominant groups are streptococci and lactobacilli. Depending on their ability to survive in an oxygen environment, gram-negative rods are divided into:
- Strict anaerobes: bacteroids (Bacteroides), Porphyromonas (Porphyromonas), fusobacteria (Fusobacterium), prevotella (Prevotella), and leptotrichia (Leptotrichia).
- Facultative anaerobes: hemophils (Haemophilus).
The microflora also includes non-pathogenic spirochetes (Treponema, Leptospira, Borrelia), mycoplasmas, and protozoa.
Briefly: answers to topical questions
Which species of streptococci dominate the resident oral microflora?
The resident oral microflora is dominated by the viridans group streptococci. The main representatives of this group are:
- Streptococcus salivarius (Streptococcus salivarius)
- Streptococcus sanguis (Streptococcus sanguis)
- Streptococcus mutans (Streptococcus mutans)
- Streptococcus milleri (Streptococcus milleri)
- Streptococcus mitis (Streptococcus mitis)
These microorganisms account for about 50% of all oral residents and are consistently detected in all samples.
Which protozoan species are part of the normal oral microflora?
The oral microbiota includes the following protozoa:
- Oral amoeba (Entamoeba gingivalis) — often detected in oral pathologies.
- Oral trichomonad (Trichomonas tenax) — a commensal of the oral cavity.
What is the physiological and pathogenetic role of lactobacilli in the oral cavity?
The physiological role of lactobacilli lies in participating in the initial colonization of the newborn's oral cavity and the formation of the microbiocenosis. Their pathogenetic role is associated with the development of dental caries:
- Primary colonization: lactobacilli are among the primary colonizers of the newborn's oral cavity.
- Development of caries: lactobacilli are leading causative agents of caries along with Streptococcus mutans.
The titer of lactobacilli serves as a criterion for assessing caries susceptibility. During fermentation, these microorganisms produce organic acids that cause demineralization of hard tooth tissues.
What stages and specific microorganisms are involved in the sequential formation of dental plaque?
Dental plaque formation occurs in several successive stages with a succession of microbial communities:
- Initial stage — formation of the acquired pellicle.
- Early colonization (day 1) — adhesion of primary colonizers, which create oxygen-free conditions. Aerobes and facultative anaerobes are involved: streptococci (S. gordonii, S. mitis, S. oralis, S. sanguinis), corynebacteria, and actinomycetes.
- Intermediate stage (days 2–4) — a decrease in the number of streptococci, growth of facultative and anaerobic actinomycetes, gram-negative cocci, and rods.
- Late colonization (after a week) — coaggregation and attachment of other bacterial species, including Porphyromonas gingivalis, Prevotella denticola, Tannerella forsythia, Treponema denticola, Capnocytophaga spp., Actinomyces spp., and Aggregatibacter actinomycetemcomitans; spirochetes appear.
Class and exam questions
How many species of microbes live in the mouth?
Traditional methods detect about 250 species, but modern molecular genetic studies reveal the presence of more than 700 species, including unculturable forms.
How does the microflora change when teeth appear?
When teeth erupt, gingival sulci are formed. This creates conditions for the proliferation of gram-negative anaerobes, which displace some of the primary colonizers.
What else to study on this topic
- Mechanisms of bacterial adhesion to oral tissues
- Structure and formation of dental plaque (biofilm)
- The principle of selective advantage in microbiocenosis
- Impact of carbohydrate diet and smoking on microflora
- Features of microflora during pregnancy
- Molecular genetic methods for microbiome diagnostics
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